In a semiconductor device, such as an IGBT (Insulated Gate Bipolar Transistor) or a power MOSFET (Metal Oxide Semiconductor Field Effect Transistor), on which a vertical semiconductor chip having an electrode on a rear surface of the chip is mounted, high Pb content solder is used for a die-bonding material in order to withstand the 260° C. reflow process in the secondary packaging. Although high Pb content solder is exempted from the regulations prohibiting the use of lead in RoHS instruction which have been enforced since 2007, Pb-free for high Pb content solder has been strongly demanded because of the increase of environment consciousness in recent years.
However, in a structure in which a Cu(copper)-based metal leadframe and a semiconductor chip using a Si (silicon) substrate are die-bonded to each other, difference of thermal expansion between base materials is large, and an alloy-based high-temperature Pb-free solder material capable of withstanding the thermal strain caused by the thermal expansion has not been found yet. As a bonding material having a reflow resistivity at 260° C. other than solder, there are an Ag (silver) paste of a resin adhesion type and a metal bond paste using Ag sintering phenomena. However, the strengths and deformabilities thereof at a joint portion are not so high as those of high Pb content solder, and therefore, their reliabilities under thermal cycle test in the case of using them in a conventional semiconductor device are lower than that of high Pb content solder, and examples where they are used for the products of a high-output semiconductor chip over 10 W have not been reported yet.
Meanwhile, for the higher reliability of a semiconductor device, a manufacturing technique of a semiconductor device for enhancing adhesion between a metal leadframe and molding compounds is known (for example, Japanese Patent Application Publication (Translation of PCT International Application) No. 2007-508448 (Patent Document 1) or IEEE TRANSACTIONS ON ADVANCED PACKAGING, Vol. 22, No. 3, pp. 398 to 406, Aug. 1999, (Non-Patent Document 1)). The manufacturing technique has a mechanism in which an alloy-based film composed of Zn (zinc) and Cr (chromium) or Zn and other metals is formed on a metal surface in an alkaline aqueous solution by an electro-plating method to make a Zn-alloy film with a whisker structure, so that the film is strongly adhered to a polymer resin by mechanical anchor effect. It is said that high-joint strength can be maintained at a joint portion even under environments such as high humidity and cyclic thermal stress and the popcorn phenomena resulting from vapor explosion in a resin-molded semiconductor device can be prevented.